Studies investigating the nature of unresponsiveness induced to heterologous erythrocytes by injection of high speed supernatants from lysed cells will be continued. The tolerogenic material will be coupled to sepharose beads which will be used as a probe of assaying serum blocking factors including possible anti-receptor antibodies, anti-tolerogen activity and antigen-antibody complexes. Induction of tolerance in utero will be attempted by injection of tolerogen during pregnancy as well as by assay for transplacental passage of serum blocking activity. The nature of the tolerogenic material will be further defined.